The quality of radiologic (X-ray) images and the extent of patient exposure to X-radiation depend to a significant degree upon X-ray dosage, i.e., the number of X-ray photons per unit area that are applied to the patient or object under investigation. Generally, the greater the applied dosage, the greater the amount of information produced in the resulting X-ray image, and hence, the better the quality of the image. From a practical standpoint, however, the intensity of the X-ray dosage used in radiological procedures is limited by the undesirability of exposing the patient to too great an amount of radiation. This limitation in many instances causes, indirectly, the production of images of less than desired quality.
Generally, to produce an image on film, the X-ray film is placed between scintillative or intensifying screens, e.g., screens having inorganic crystalline phosphors embedded therein. The screens act to stop and absorb the X-rays, and to convert them to visible light. The screens also act to transfer the visible light (i.e., allow the visible light to escape) to the X-ray film. When an X-ray photon which has interacted with matter under investigation strikes a phosphor in the screen and is absorbed by the phosphor, the phosphor scintillates and gives off visible quanta of light. Some of this light escapes from the screen and strikes the film, thereby exposing the film at the point of incidence and producing an optical image thereon.
Conventional X-ray intensifying screens, as shown, for example, in FIGS. 1A and 1B, are generally used in pairs, one screen above the film, and one screen below the film, sandwiching the film. Such paired usage prevents, to a significant extent, lateral diffusion of the visible light photons and consequent loss of resolution of the optical image. A screen may be thin (about 125 .mu.m in thickness), or thick (about 200 .mu.m in thickness). A thick screen typically has a large quantity of phosphors interposable in the path of the X-ray photons and provides substantial X-ray photon-stopping absorption and conversion power. Thin screens, on the other hand, typically have a smaller quantity of phosphor particles that are interposable in the path of X-ray photons than do thick screens, thereby causing less diffusion (scatter) of light photons than do thick screens (and, hence, providing better image resolution or detail). However, thin screens have lower "speed" capabilities, i.e., poor photon stopping, or absorption capabilities than do thick screens, and requires greater patient exposure or dosage than thick screens. Thick screens typically have high speed capabilities, i.e., good photon-stopping and absorption capabilities (hence, provides less dosage to the patient) but poorer image resolution. When an image with good resolution is desired, therefore, a pair of thin image intensifying screens such as shown in FIG. 1A is generally used to produce the image. When a lesser-dosage, higher-speed image is desired, i.e., an image where a large percentage of the applied X-ray photons are utilized (stopped, absorbed, converted to visible light, and allowed to escape to expose the film), a pair of thick screens such as shown in FIG. 1B is used. To produce an image using thick screens, therefore, less radiation and hence, less dosage to the patient is required than for thin screens, but a greater amount of diffusion occurs, resulting in images of poorer resolution.
Typically, each screen of the pair, whether thin or thick, is constructed of the same phosphor or scintillator substance and have the same thickness. Generally, this phosphor substance is particulate in form, and is sprayed onto a backing material to form the screen. Another phosphor or scintillator material, sodium doped cesium iodide, CsI(Na), is growable in pillar (bulk) form or in needle (fiber) form, but is unstable (i.e., it decays, and its scintillative properties become degraded) when used under ambient conditions (i.e., at a temperature of about 20-25 degrees centigrade, and at a relative humidity of about 75%). The fact that one scintillator material (e.g., sodium doped cesium iodide crystals) is growable in needle form, does not suggest or otherwise make it obvious that another scintillator material (e.g., thallium doped cesium iodide crystals) is also growable in needle form. Also, because CsI(Tl) crystals are growable in pillar form does not suggest or otherwise make it obvious that said crystals are also growable in needle form. Actually, the addition of a substance to a compound, in certain concentrations, may cause no crystal growth, or cause a reversion from needle growth to another type of growth such as malformed dendritic growth as described, for example, on page 138 of the article by B. J. Mason entitled "Ice", appearing in The Art and Science of Growing Crystals, edited by J. J. Gilman and published by John Wiley and Sons, Inc., New York, 1963. Furthermore, even if crystal growth in needle form is achieved for a scintillator material, there is no guarantee that such a material will be stable at ambient conditions as indicated above by the instability of CsI(Na).
What is needed, therefore, is a novel screen pair which includes scintillator material that is stable and does not decay under ambient conditions, and which provides substantially as good an image resolution as thin screens, and has substantially as high a speed capability and requires substantially as little dosage as thick screens. What is also needed is a method of producing the novel screen pair.